1. Field of the Invention
This invention relates to a stepless or continuously variable automatic transmission to be mounted between an engine and a drive wheel axle of a motor vehicle.
2. Description of the Prior Art
The front-engine/front-drive type motor vehicles have come to employ a stepless automatic transmission which has its power transmission shaft divided into three parts as shown in FIG. 1 for the purpose of reducing its axial dimension from the standpoint of improving the mountability of the transmission. As seen in the figure, a first shaft 10 which is disposed coaxially with the engine output shaft is provided with an input pulley 52. A second shaft 20 which is disposed parallel with the first shaft is provided with an output pulley 56 and a helical output gear 59 at its output end, and, in order to ensure its rigidity, supported at three points by the transmission case including a torque converter case accommodating the fluid joint, a transmission case fastened to the torque converter case and a center case fastened in position between the torque converter and transmission cases. The third shaft is disposed parallel with the second shaft and provided with a speed-reducing idler gear. In such a conventional stepless automatic transmission, it is necessary to determine the relative axial positions of the input and output pulleys with a high degree of accuracy to ensure higher efficiency and durability of the belt. In the above-described case, a single ball bearing is employed to determine the axial positions of the pulleys and to support the large thrust force imposed by the helical gears, so that there have been problems that the ball bearing has a short service life and the bearings at the three supporting points are required to have a high degree of concentricity, necesitating simultaneous machining of the bearings to improve the accuracy of the machining operation. This is obviously reflected by low productivity and high production cost.